The spray-painted letter “m” — for the Arabic word “mukawama,” meaning resistance — was part of a campaign by Kitaeb al-Mosul, an underground opposition group in the northern Iraqi city that released a video detailing their efforts this month.

The Islamic State reacted with swift brutality, executing three young men it accused of being involved. The militants released their own video showing the men kneeling in orange jumpsuits before being shot in the head. The letter “m” was sprayed on the wall behind them, a reference to their alleged crime. A spray can lay on the ground beside them, surrounded by blood.

In recent months, the Islamic State has carried out more arrests and executions such as these in a sign of desperation as it faces the prospect of losing Mosul, according to reports from inside the city.

Mosul is the largest city under Islamic State control and is central to its narrative of having restored the Islamic caliphate. It was less than a month after Mosul fell in June 2014 that Abu Bakr al-Baghdadi appeared in the mosque there and called on Muslims to follow him.

The recapture of the city would be a significant step toward depriving the Islamic State of its territory and forcing the group back into an insurgency, U.S. and Iraqi officials say. That is only a matter of time, they add.

Prime Minister Haider al-Abadi has pledged to retake Mosul by the end of the year, and the Iraqi air force dropped 7 million leaflets on the city last week telling residents to prepare for the “zero hour.”

As Iraqi forces — and the U.S. troops advising them — move closer, making the recently recaptured Qayyarah Air Base, 25 miles south of Mosul, a logistical hub for the impending battle, the Islamic State has also been making preparations.

“Daesh is weaker in Mosul, but it is using methods of oppression like random arrests to try and show it is still in control,” said a representative of Kitaeb al-Mosul. Daesh is an alternative name for Islamic State. He spoke on the condition of anonymity for security reasons. He described the atmosphere in the city as “tense” and said the militants were in a state of “confusion.”

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